Heat insulating lining for electric furnaces



Jan. 20, 1970 P. H. H. BISHOP E AL 3,491,192

HEAT INSULATING LINING FOR ELECTRIC FURNACES Filed May 24, 1968 3Sheets-Sheet 1 FIG. I

HEAT INSULATING LINING FOR ELECTRIC FURNACES Filed May 24, 1968 I 1970P. H. H. BISHOP ET AL 5 Sheets-Sheet 2 I I l/ 1 I I l I 3,491,192 HEATINSULATING FOR mums ELECTRIC FURNACES Filed May 24, 1968 Jim. 1970 P. H.I-t BISHOP ET AL 5 Sheets-Sheei 3 FIG. 3,

United States Patent 3,491,192 HEAT INSULATING LINING FOR ELECTRICFURNACES Peter Harold Howard Bishop, Farnham, and Kenneth FrancisRogers, Farnborough, England, assignors to National Research DevelopmentCorporation, London, England Filed May 24, 1968, Ser. No. 731,784 Claimspriority, application Great Britain, May 24, 1967, 24,07 8/ 67 Int. Cl.H05b 3/62 U.S. C]. 13-20 6 Claims ABSTRACT OF THE DISCLOSURE A furnaceis made up of an outer case 1 of electrically insulating hardboard and alining comprising a mass of spirally coiled rolls 2 of carbon fibrespacked in to define and thermally insulate an inner charge-receivingspace and to aiford support for a charge which can be placed on a carbonhearth 4. The axes of the rolls are substantially normal to thecharge-receiving-space boundaries which they define. The rolls are madeof carbonised cotton wool; they may be pyrolisedat least in part-insitu, secured to the walls of the case by ordinary adhesive.

Tubular electric resistance-heating elements 6 are mounted in insulatingtubes 11.

The walls of the case are of sandwich form, the middle layer 9 having anopening 9a forming a manifold receiving inert gas from a pipe 5 andcommunicating with the interior of the furnace through the pores of thewhole mass of carbon fibre rolls. A pipe 12 for discharge of gas fromthe furnace passes through one of the heaters 6.

In one conventional construction of large furnace for operating atsomewhat high temperatures-Le. 1500 C. and abovean outer casing (usuallyof steel) encloses an inner furnace lining of carbon blocks supportedfrom the outer casing. The lining is separated from the outer casing bya wide gap which is packedbetween the supports-with thermal insulationin the form of carbon granules or felt and usually ordinary lampblack.Such a furnace is used of course for heating a charge in a vacuum or anon-oxidising atmosphere. Both in manufacture and during operation thedirty nature of the lampblack and the fact that it is granular makes itvery difficult to handle and causes soiling of the surroundings wheneverthe insulating lining has to be opened up for inspection or repair. Thelampblack continuously tends to settle and needs to be topped up fromtime to time. The supports extending across the heat-insulating gap fromthe outer casing to the inner lining conduct some waste heat across thegap and lower the efliciency of the furnace.

With a view to overcoming some of the aforesaid disadvantages thepresent invention is concerned with a cocoon construction ofheat-insulating fibres which replaces the lampblack, can partly orwholly eliminate the supporting rods passing across the heat-insulatingspace and hence can reduce heating of the casing and loss of efliciencyby conduction through the supports, and which can wholly eliminate thecarbon slab lining except perhaps for a heart-h on which the charge canlie. Thus according to the invention a heat-insulating packing in afurnace is a mass of spirally coiled rolls of heat-insulating fibrestightly packed in to define and thermally insulate an innercharge-receiving space, and also affording support for a charge in thefurnace. Preferably the rolls are stacked with their axes normal to thepart of the outer casing which supports them and hence normal to thesurfaces of the charge-receiving space which they define. Such spirallycoiled rolls of fibrous material can serve as structural elementssupporting substantial loads as set forth in our prior British patentspecification No. 921,061, and thus this fibre packing and lining servesboth as heat insulation and support for the weight of the charge. Inparticular the packing is of carbon fibres, these being good thermalinsulators, non-friable andunlike carbon in the form of lampblackpowder-reasonably clean to the touch. A cheap and useful material iscotton waste which has been carbonised by heating to a carbonisingtemperature in a non-oxidising atmosphere. Textile waste of othernatural or synthetic fibres may be used.

One particular construction according to the invention is illustrated bythe accompanying drawings of which:

FIG. 1 is a front view of a furnace with the door open FIG. 2 is asection on IIII of FIG. 1 and FIG. 3 is a section on III-III of FIG. 2.

An outer casing 1 made up of bottom, top and side plates and ablackplate, but with the front left open for inserting the charge intothe furnace, is lined with a filling of closely packed spirally coiledrolls 2 of carbon fibre, some rolls being stacked on the bottom of ordepending from the top of the casing with their axes 2a substantiallyvertical and others being stacked against the sides and back of thecasing with their axes 2b substantially horizontal. The inner ends 212of these rolls 2 form the faces of and so define an innercharge-receiving space, each face thus being substantially normal to theaxes 2a or 2b of the rolls forming it. The rolls are chosen ofsufficient length to provide the necessary heat insulation" between thecharge and the outer casing; the drop in temperature along each rollwill be sufficient for the temperature of their outer ends 2d to be somuch lower than the temperature within the inner space that the rollsmay be held securely by attaching these outer ends 2d to the innersurfaces of the casing walls by ordinary adhesive.

Each roll 2 may be made by taking a commercially available roll ofcotton wool, or rolling a sheet of any fibrous carbonisable material,and heating it to a carbonising temperature in an inert or at leastnon-oxidising atmosphere, taking it up to what will be at least theworking temperature of the furnace.

The walls of the casing 1 may be of metal, as is conventional, but sincethey will be at low temperature they may be hard boards of an electricalinsulating material. They are built up as described later.

The front access opening can be closed by the hinged door 3 (FIG. 2)which is built up of a composite plate carrying a set of the fibre rolls2, similarly to the walls 1. The slightly soft and yielding nature ofthe fibrous rolls greatly reduces risk of jamming of the door andconsequent damage due to a slightly badly fitting door.

Resting on the rolls 2 standing up from the bottom plate of the casing 1are a number of carbon blocks or tiles 4 forming a charge-receivinghearth; the weight of the hearth, loaded by a charge, is taken by axialcompressive loading of the spiral rolls 2. If the nature of any chargein the furnace be such that by its weight it imposes some load on thesides and back of the furnace this load will also be taken by axialcompression of rolls 2 One wall of casing 1 has an opening for pipe 5for the admission of the gas-in particular an inert gas, or a gaseousmixture which does not include oxygenwhich may be required as theatmosphere in the furnace for any particular process or for theadmission of purge gases before changing from one atmosphere to another.

Each wall of the casing 1 is made up of inner and outer plates 7 and 8between which is sandwiched a plate 9 pierced by an opening 9a forming amanifold. The pipe 5 opens into the manifold in one of the plates 9.Also in communication with each manifold is a set of holes 9b in plates7 and 9 opening onto the rear end faces 2d of the rolls so that gas maypass between the manifold and the rear of each mass of fibre rolls. Themanifold 9a in each side of the casing 1 communicates with the manifoldin a neighbouring side wall through a short pipeelbow 10 passing throughthe middle plates 9 of each side; similarly one side wall communicateswith the interior of the top wall and one side also communicates withthe interior of the bottom wall. Thus pipe 5 is in communication withall the manifolds 9 and thence with the rear of all the masses of fibrerolls on the casing 1.

Pipe 5 can be used to fill the furnace with the desired atmospherebefore use. When, after use, the furnace is opened up by opening thedoor 3, the temperature inside may still be high enough for incomingatmospheric air to ignite some partof the carbon fibres; accordinglypipe 5 is again used to introduce pressurised cooling protective gasstarting before the furnace is opened, the gas being inert or at leastincapable of supporting combustion. This gas can pass through the poresof the whole fibrous mass into the interior space of the furnace andoppose entry of air into the fibrous mass. When the furnace is beinginitially filled with gas the furnace will not be so completely sealedas to prevent leakage of the air which is being driven out, or there maybe an escape through a further pipe 12 provided as describedhereinafter. To guard against risk of displacement, wear or damage ofone roll putting one of the holes 9b into direct communication with theinterior space and thereby draining off all the gas from the other holes9b, each of these holes communicates with its associated manifold 9athrough a narrow slit 9c in the plate 9, the drop of pressure along thisslit being comparable with that in the pores of the fibrous mass.

Preferably, as shown, the door 3 is constructed similarly to the wallsof the casing 1 so that inert gas can be passed through the fibrous massthereon. To permit movement of the door between the open and the closedposition the pipe-elbow 10a between the top of the casing and the dooris flexible. Gas passing through the pores in the bottom plate of thecasing 1 will enter the inner space through the gaps between theindividual blocks of the hearth 4.

The furnace is electrically heated by tubular nonmetallicresistance-heating elements, two of which are shown in external view at6. Since the carbon-fibre rolls are usually electrically conductive theelements must be insulated from them. Accordingly, two bushes 11 ofelectrical insulating material pass through the mass of fibrous fillingon the backplate of the furnace. The elements 6 pass through entry holesin the backplate, through the bushes 11, and extend to the front of thecharge-receiving space as rear-supported cantilevers. Preferably theseelements are of high resistance, taking comparatively low current at avoltage of the order of the voltage of supply mains.

A further pipe communicating directly with the interior of thecharge-receiving space may be desired. This may for example be necessaryfor continuous escape of gases which may be driven off by some heatingprocesses, and such pipe may be required to be kept hot by proximity tothe source of heat; such a pipe is shown at 12 connected to the interiorof one of the tubular heaters 6 and communicating with the furnaceinterior through openings 6a. A seal 13 is provided between the elements6 and the backplate around the pipe 12 to reduce leakage of gas by anypath other than through pipe 12.

In making the furnace the fibre rolls 2 may be assembled therein in theuncarbonised or in an only partially finished state and then becarbonised or matured by pyrolysis in situ.

We claim: I

1. A furnace made up of an outer case, heating means carried by saidcase and a lining within said case, said lining comprising a mass ofspirally coiled rolls of heatinsulating fibres packed into said case todefine and thermally insulate an inner charge-receiving space and toafford support for a charge.

2. A furnace according to claim 1 wherein said lining comprises rollsspirally coiled around axes substantially normal to thecharge-receiving-space boundaries which they define.

3. A furnace according to claim 2 wherein said rolls are of carbonmaterial.

4. A furnace according to claim 3 wherein said rolls are of carbonisedcotton-wool.

5. A furnace according to claim 3 comprising gas connections to the rearof said rolls for the passage of nonreacting gas between said connectionand the chargereceiving space by way of the pores in the whole mass ofrolls.

6. A furnace according to claim 1 wherein said heating means includes ahigh-resistance electric heating element.

References Cited UNITED STATES PATENTS 1,517,820 12/1924 Bong -952,657,247 10/ 1953 Bretschneider 13-20 X 3,213,177 10/1965 Diefendorf1320 X BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, AssistantExaminer US. Cl. X.R. 13-31

